The objective of the proposed research is to investigate the mechanism of action of thyroid hormones at the cellular level through a study of the relationship between hormone-receptor interactions, hormone action and hormone metabolism. This work will be carried out on developing Xenopus laevis tadpoles during the time that they acquire sensitivity to thyroid hormone and finally undergo metamorphosis in response to the actions of the hormones. This research will concentrate on the mechanism of action of thyroid hormones on tadpole liver and tail tissues during spontaneous metamorphosis and on isolated tail and liver tissues cultured in vitro in the presence of thyroid hormones.